Ever heard of CheaperThanDirt? They have highly rated AR-15 mags for 10 bucks here:
http://www.cheaperthandirt.com/MAG065-36.html
If I had an AR that's probably what I would buy. Brownells boasts a lot out of their mags but I don't know much about them. Still, they too have fairly good ratings. Brownells have steel ones but they are more expensive and more or less unnecessary. Link to Brownells mags:
http://www.brownells.com/.aspx/pid=2...OUND_MAGAZINES